Yeah, that's kind of what I was thinking about. My method was a little less refined and involved a lot of "glowing area measurement", but it's essentially the same principle.
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PolynomialApr 18 '12 at 17:34

3 Answers
3

Choose a component of blood that is replenished slowly [from liver or marrow etc]. That takes time to replenish. Red blood cells are probably OK. Some easily measurable and slowly replenished component of blood. Let's call this component of blood G.

Take small blood sample and measure level of G, denote result G1.

Drain N ml of blood, ands refill same amount back with infusion of normal saline.
N can be taken as 10% of estimate of amount of blood.

Take small blood sample, measure level of G, denote result G2.

G2 will be smaller than than G1.

Now from G1, G2, and N it is easy to calculate organism's amount of blood.
You can derive exact formula yourself.

How long would you wait between samples given that you would presumably have to wait for "G" to diffuse throughout the entire blood stream?
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Rory M♦Apr 14 '12 at 13:03

@Rory M: I would suspect diffusion in the blood stream to be almost negiglible. If I remember correctly it takes ~1 min for a complete recirculation of blood in a human, and for smaller animals it would be even less than that.
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nicoApr 14 '12 at 16:38

Your model isn't very accurate since it depends on making a "rough estimate" of the amount of blood before making any measurements. That's a huge bias that can't be overcome later. I also think your suggestion of removing 10% of the estimated blood volume isn't commendable. What happens in a case where you over-estimated the blood volume? Your model works, but only for an estimation. I think you should provide the equation for any readers who aren't very mathy.
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CHMApr 16 '12 at 3:59

@CHM, I believe that N doesn't need to any particular value. The dilution series will allow one to back-calculate what x% N consist of.
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bobthejoeApr 16 '12 at 5:41

Polynomial gives a good hint. By adding in a known tracer of known amount that should only circulate in the blood stream, the concentration of the tracer when completely distributed will give the volume of distribution.

If the tracer only stays in the bloodstream (and that's a fairly large IF), then the VD will be equal to the VBlood. However, as many pharmacologists know, interactions with proteins and cells can skew that number. The blood volume is also reduced by the volume of the blood cells. However, using Andrei's method of dilution, one can merely swap the cell counts with the tracer concentration.

How it is done according to my bioengineering textbook is injection of a water-soluble tracer into the heart or aorta, then 'listening' for the tracer somewhere else in the bloodstream. If you inject into the atrium and listen at the aorta, you can get cardiac output as well. It takes a minute or so, and gets a measure of the amount of fluid circulating, not counting blood that's trapped or basically not circulating(veins from leg muscles while sleeping, compartment syndrome, etc) while counting fluid that freely exchanges with the blood. Not technically accurate, but gives extra information.
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ResonatingAug 12 '14 at 21:21

The same book described measuring cardiac output using ice cold saline injected directly into the atrium as a tracer, and it was written quite some time ago(70s iirc). On the whole I trust it as far as I can throw it.
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ResonatingAug 12 '14 at 21:23